1. Field of the Invention
The present invention relates to a semiconductor device having a solid-state image sensor.
2. Description of the Background Art
Recently, a type of solid-state image sensor which uses an amplification-type sensor is suggested. This type of device is characterized in that the light signal detected in the photoelectric conversion storage portion is amplified in the close vicinity of the photoelectric conversion storage portion.
FIG. 9 is a diagram showing the circuit configuration of a semiconductor device having a CMOS (Complementary Metal Oxide Semiconductor) type image sensor as a solid-state image sensor. As shown in FIG. 9, unit pixels or unit cells, C, are arranged in a matrix, where the individual cells C are connected to a vertical shift register VS and a horizontal shift register HS.
Each unit cell C has a photodiode PD, a transfer switch M1, a reset switch M2, an amplifier M3, and a select switch M4. The photodiode PD serves as the photoelectric conversion storage portion which converts the incident light into an electric signal and stores the generated charge. The transfer switch M1 serves to transfer the converted electric signal to the amplifier M3; the transfer switch M1 is controlled by signal from the vertical shift register VS. The reset switch M2 serves to reset the signal charge and the amplifier M3 serves to amplify the electric signal.
The transfer switch M1, the reset switch M2, the amplifier M3, and the select switch M4 are each formed of an MOS transistor.
FIG. 10 is a top view which specifically shows the structure of the region R shown in FIG. 9. FIG. 11 is the cross-sectional view taken along the line XIxe2x80x94XI in FIG. 10.
As shown in FIGS. 10 and 11, an element isolation insulating layer 103 is formed by LOCOS (Local Oxidation of Silicon) in the surface of a P-type semiconductor substrate 102. The photodiode PD, the transfer switch M1, and the reset switch M2 are closely arranged in the surface of the P-type semiconductor substrate 102.
The photodiode PD is formed of a PN junction between the P-type semiconductor substrate 102 and an N-type impurity region (an N-type active region) 104. A P-type impurity region (a P-type active region) 105 is formed over the N-type impurity region 104 (or in the vicinity of the surface of the P-type semiconductor substrate 102). This P-type impurity region 105 is formed to such a depth that the depletion layer at the PN junction between the P-type semiconductor substrate 102 and the N-type impurity region 104 will not reach it. The role of the P-type impurity region 105 will be described later.
The transfer switch M1 has an N-type source region 104, an N-type drain region 106a (an N-type active region, which is shown as FD (Floating Diffusion) since it may come in a floating state during operation), and a gate electrode layer 108a. The N-type source region 104 and the N-type drain region 106a are formed at a certain distance in the surface of the P-type semiconductor substrate 102. The gate electrode layer 108a is formed on the surface of the P-type semiconductor substrate 102 in the part interposed between the N-type source region 104 and the N-type drain region 106a, with a gate insulating layer 107 provided therebetween. The N-type impurity region 104 of the photodiode PD and the N-type source region 104 of the transfer switch M1 are the same region, though they were separately referred to as parts of different components.
The reset switch M2 has a pair of N-type source/drain regions 106a and a gate electrode layer 108b. The pair of N-type source/drain regions 106a are spaced at a certain distance in the surface of the semiconductor substrate 102. The gate electrode layer 108b is formed on the region between the pair of N-type source/drain regions 106a with a gate insulating layer (not shown) interposed therebetween. The N-type drain region 106a of the transfer switch M1 and one of the N-type source/drain regions 106a of the reset switch M2 are the same region, though they were separately referred to as parts of different components.
In the CMOS type image sensor shown in FIGS. 10 and 11, the charge generated in the photodiode PD is transferred to the N-type drain region 106a through a channel formed in the surface of the P-type semiconductor substrate 102 right under the gate insulating layer 107 of the transfer switch M1.
In the surface of the P-type semiconductor substrate 102, the area near the edges of the element isolation insulating layer 103 is susceptible to defects caused by stresses produced by formation of the element isolation insulating layer 103. Also, when the gate electrode layer 108a is formed by etching, the etching may damage the surface of the P-type semiconductor substrate 102 and cause defects. Furthermore, impurity implantation for forming the active regions, e.g. the N-type drain region 106a, is also likely to damage the surface of the P-type semiconductor substrate 102 and cause defects.
Thus part of the signal charge generated in the photodiode PD flows through the defects to cause a leakage current. The leakage current reduces the amount of charge transferred to the N-type drain region 106a, which reduces the sensitivity of the solid-state image sensor and deteriorates the characteristics of the pixels.
The P-type impurity region 105 is formed for the purpose of suppressing such leakage current. That is to say, even when defects are present in the vicinity of the surface of the P-type semiconductor substrate 102, the presence of the P-type impurity region 105 causes a PN-junction depletion layer to form between the P-type impurity region 105 and the N-type impurity region 104, which isolates the charge storage region in the N-type impurity region 104 from the defects, thereby preventing the charge generated in the photodiode PD from being taken into the defects. The leakage current caused by defects can thus be suppressed.
However, the formation of the P-type impurity region 105 alone cannot sufficiently suppress the leakage current. As stated above, in the surface of the P-type semiconductor substrate 102, defects are likely to be caused not only in the photodiode PD, but also in the vicinities of the edges of the element isolation insulating layer 103 and in the vicinity of the channel layer. Therefore leakage current may occur through defects formed in these portions.
An object of the present invention is to provide a semiconductor device having a solid-state image sensor with reduced leakage current.
According to the present invention, a semiconductor device includes: a semiconductor substrate of a first conductivity type; a first active region; a second active region; a control electrode; and a buried channel layer. The first active region is provided in a surface of the semiconductor substrate and has a second conductivity type which is different from the first conductivity type. The second active region is provided in the surface of the semiconductor substrate at a distance from the first active region and has the second conductivity type. The control electrode is provided on the surface of the semiconductor substrate in a part interposed between the first and second active regions. The buried channel layer is provided in the semiconductor substrate under the control electrode and has the second conductivity type. Further, the buried channel layer is in contact with both of the first and second active regions. The semiconductor substrate and the first active region constitute a photodiode which is part of a solid-state image sensor. The control electrode and the first and second active regions constitute a transistor which is part of the solid-state image sensor. Further, the buried channel layer has a lower impurity concentration than the first and second active regions.
According to the present invention, the semiconductor device has a buried channel layer which is in contact with both the first and second active regions. Accordingly the channel for transferring a charge generated in the photodiode can be formed in the vicinity of the interface between the buried channel layer and the semiconductor substrate. That is to say, it is possible to avoid channel formation in the defect-prone semiconductor substrate surface, thereby realizing a semiconductor device having a solid-state image sensor with reduced leakage current.